Gating system

ABSTRACT

A gating system for adding an alloying material to a molten base metal in immediate connection with a casting process. The gating system has a runner ( 2 ) having an inlet ( 7 ) whose cross-sectional area is throttled, a reaction chamber ( 4 ) whose sectional area varies along the height of the reaction chamber ( 4 ) as a function of the teeming rate, and a pressure and mixing chamber ( 5 ) which is connected after the reaction chamber ( 4 ) and provided with a partition ( 9 ). This results in a constant alloying material content of the metal being obtained at a varying teeming rate during the casting process.

FIELD OF THE INVENTION

[0001] The present invention relates to a gating system for adding analloying material to a molten base metal in immediate connection with acasting process.

BACKGROUND ART

[0002] When casting iron alloys, a modification of the iron can takeplace prior to casting by adding different alloying materials to thepouring ladle or to a special treatment ladle. A different manner is tosupply alloying materials successively during the actual castingprocess. One example is the Inmold process. In the Inmold process whichis used for manufacturing nodular iron alloys a reaction chamber isformed in the mould drag. At one edge the reaction chamber is connectedto the sprue of the gating system via a short duct and at the other edgeto a duct leading to the inlets to the casting. A certain amount ofcrushed FeSiMg alloy containing about 5% magnesium is placed in thereaction chamber. When casting, the iron flows into the chamber, theFeSiMg alloy melting on the surface and being gradually dissolved in theiron flowing through the reaction chamber. About 0.35% magnesium isdissolved in the iron which gradually fills the casting cavity. In thesolidification, carbon is separated in the form of graphite as nodules,which characterises nodular iron. If the amount of magnesium is too low,the iron can wholly or partly solidify as grey cast iron, which hassignificantly lower strength. To prevent this, the reaction chamber issomewhat oversized. What is essential in the manufacture of nodular ironis that the amount of magnesium is not allowed to be lower than acertain minimum level. Higher contents than the standard value-do notproduce any considerable detrimental effects.

[0003] The sectional area of the reaction chamber is decisive of theamount of magnesium that is dissolved in the iron at a given teemingrate (kg/s). The sectional area is dimensioned to an average teemingrate and is constant along the height of the reaction chamber. If theteeming rate is not constant during the casting process but decreases,this results in the magnesium content of the iron gradually increasingin inverse proportion to the teeming rate. This takes place, forinstance, if the delivery head in casting decreases by part of thecasting cavity being positioned above the parting line of the mould.When manufacturing nodular iron this does not cause any major problemsas mentioned above, since it is possible to operate with safety marginsfor the addition of magnesium.

[0004] However, problems arise if compacted graphite iron is to bemanufactured by the Inmold process. Compacted graphite iron ischaracterised in that the carbon dissolved in the iron is separated asvermiform graphite particles, not as spheres as in nodular iron, or asthin flaky structures as in grey cast iron. The compact graphite form isan intermediate form which only arises within a very narrow magnesiumrange which is dependent on, inter alia, the material thickness. Atypical range is 0.01 to 0.013%. Using the conventional Inmold processwhere the sectional area of the reaction chamber is constant, themagnesium content can increase from 0.01 up to 0.02% if the teeming rateduring the later part of the casting is reduced to half the initialrate. As a result, the iron having the higher magnesium content willcontain a small amount of compacted graphite and a large amount ofnodular graphite, i.e. a mixture of compacted graphite iron and nodulariron.

[0005] Another problem in the manufacturing of compacted graphite ironis that the lower limit of magnesium is dependent on the nucleationstate of the base iron. The nucleation state can be measured indirectlyusing different methods, for instance thermal analysis, and for optimalconditions, it would be necessary to vary the percentage of magnesium inthe iron in relation to the nucleation state. This is not possible withthe traditional Inmold process.

[0006] One more problem of the traditional Inmold process is that partof the first iron that reaches the reaction chamber owing to the kineticenergy passes into the duct from the reaction chamber without havingbeen in immediate contact with the alloying material. The reactionchamber is not completely filled with metal until after a few seconds.This means that the first iron which flows into the casting cavity mayin some cases have too low an alloying material content.

SUMMARY OF THE INVENTION

[0007] The object of the present invention is to provide a gating systemfor obtaining a constant alloying material content in the metal at avarying teeming rate during the casting process.

[0008] This object is achieved by a gating system of the type stated byway of introduction, which has the features defined in claim 1.

BRIEF DESCRIPTION OF THE DRAWING

[0009] The invention will now be described in more detail by way ofexample and with reference to the accompanying drawing, in which

[0010]FIG. 1 is a perspective view and shows a preferred embodiment ofthe gating system of the invention;

[0011]FIG. 2 is a sectional view and shows the first part of the gatingsystem; and

[0012]FIG. 3 is a sectional view and shows the second part of the gatingsystem.

DESCRIPTION OF A PREFERRED EMBODIMENT

[0013]FIG. 1 shows an embodiment of a gating system for production ofcompacted graphite iron. The base iron is supplied to the system via apouring ladle or founding furnace via a pouring cup and a sprue 1. Arunner 2 is connected to the sprue 1. The first part 7 of the sprue (seeFIG. 2) is of a cross-section which in prior-art manner has beendimensioned to obtain the desired flow and, thus, the desired durationof casting for the component which is to be cast. The second part of therunner 2 is formed with a cross-section which is three times that of thefirst part 7. In the second part of the runner 2 a connecting duct 3 isconnected perpendicular to the reaction chamber 4. The runner 2 projectspast the connecting point of the connecting duct 3. The extension 8makes the flow stabilise in the sprue 1 before the base iron via theconnecting duct reaches the reaction chamber 4. The cross-section of theconnecting duct 3 is adjusted to the volume flow so that the rate to thereaction chamber 4 is less than 500 mm/s. The width of the connectingduct 3 is equal to the width of the reaction chamber 4.

[0014] The reaction chamber 4 is formed with a square cross-section andits sectional area on different levels is calculated according to theformula;

Sectional area per level (cm ²)=(Q×DMg/100)/F

[0015] Q=Metal flow (g/s)

[0016] DMg=Desired magnesium content (%)

[0017] F=Factor for taking up magnesium from the reaction chamber(g/cm²/s)

[0018] The alloying material, for instance FeSiMg having a particle sizeof 1-3 mm, is in known manner placed in the reaction chamber 4. Duringcasting, metal flows through the upper part of the reaction chamber 4,and the alloying material melts gradually and is dissolved in the iron.

[0019] The flow of metal during the casting time is calculated in knownmanner with the aid of the current efficient pressure head at each pointof time or by carrying out a computer-aided flow simulation. The heightof the reaction chamber 4 is calculated in known manner in relation tothe total amount of magnesium alloy and the density thereof as well asthe sectional areas. The height of the upper part of the reactionchamber 4 is increased by at least the height of the connecting duct 3.

[0020] A pressure and mixing chamber 5 is arranged on the opposite sideof the connecting duct 3 to the reaction chamber 4. The connection areato the reaction chamber 4 is equal to or greater than the area of theconnecting duct 3. The pressure and mixing chamber 5 is divided by apartition 9 (see FIG. 3). The purpose of the partition 9 is to ensurethat the reaction chamber 4 is completely filled with metal and ispressurised before metal is allowed to flow out in the outlet duct 6leading to the casting cavity. The height of the partition is calculatedaccording to the formula

Height of partition (mm)=30+3×height of the inlet to the reactionchamber

[0021] The height of the pressure and mixing chamber 5 is equal to theheight of the partition 9 plus the height of the connecting duct 3 tothe reaction chamber 4. The volume of the first part of the pressure andmixing chamber 5 is half the volume of the reaction chamber 4.

[0022] The outlet duct 6 from the pressure and mixing chamber 5 has across-sectional area which is equal to or greater than that of theconnecting duct 3. The outlet duct 6 is connected either direct or via aceramic metal filter to the casting cavity in known manner.

[0023] According to the invention, a desired variation of the magnesiumcontent of the iron is obtained, to achieve an optimal level in relationto the metallurgical status of the base iron and the cooling rate of thecasting component, in three ways.

[0024] First, the teeming rate, i.e. the flow through the reactionchamber 4, can be varied. Experiments have demonstrated that the take-upof magnesium from the alloying material in the reaction chamber 4 for agiven alloying material is a function of exposed alloying material areaand the time of contact with the liquid base iron. The take-up ofmagnesium as g Mg/cm² of reaction chamber area and second is establishedempirically by casting experiments. A normal value of commercial FeSiMgalloys containing about 4% Mg is 0.015 g/cm² of reaction chamber areaand second. At a given sectional area, the take-up of magnesium cantherefore be varied by varying the flow through the reaction chamber 4.In practice, this can easily be carried out by varying the casting timeand, thus, the flow in kg/s by changing the throttle of thecross-sectional area at the beginning 7 of the runner. Most castingcomponents withstand a variation of the casting time of +/−20% withoutany risk of casting defects. This makes it possible to vary themagnesium content within sufficiently wide limits in order to correctfor variations in the base iron which affect the nucleation process ofgraphite.

[0025] Second, also an increase or decrease of the sectional area of thereaction chamber 4 at different levels allows a variation of themagnesium content. This can be carried out by using exchangeablepatterns for the reaction chamber 4 or in some other manner varying thesectional area of the chamber. An increased area increases the take-upof magnesium and vice versa.

[0026] Third, the reaction chamber can be filled with a mixture of twodifferent magnesium alloys with different dissolving capacity in orderto vary the magnesium content of the iron. The dissolving capacity maybe varied by varying the particle size of the magnesium alloy and/or byvarying the magnesium content. The mixture is adjusted to the need formagnesium as a function of the properties of the base iron in the formof nucleation capacity, degree of oxidation and design and solidifyingrate of the casting component.

1. A gating system for adding an alloying material to a molten basemetal in immediate connection with a casting process for achieving aconstant alloying material content of the metal at a varying teemingrate during the casting process, comprising a runner (2) having an inlet(7) and a reaction chamber (4), characterised in that thecross-sectional area of the inlet (7) is throttled, that the sectionalarea of the reaction chamber (4) varies along the height of the reactionchamber (4) as a function of the teeming rate, and a pressure and mixingchamber (5) is connected after the reaction chamber (4) and has apartition (9).
 2. A gating system as claimed in claim 1, wherein thethrottling of the cross-sectional area at the inlet (7) of the runner(2) is variable.
 3. A gating system as claimed in claim 1 or 2, whereinthe size of the sectional area of the reaction chamber (4) variesproportionally to the teeming rate.
 4. A gating system as claimed in anyone of claims 1-3, wherein the sectional area of the reaction chamber(4) is variable by means of exchangeable patterns.
 5. A gating system asclaimed in any one of claims 1-4, wherein the cross-sectional area ofthe outlet of the runner is at least 3 times the cross-sectional area ofthe inlet (1) of the runner.
 6. A gating system as claimed in any one ofclaims 1-5, wherein the outlet of the runner is connected perpendicularto the connecting duct (3) to the reaction chamber (4).
 7. A gatingsystem as claimed in any one of claims 1-6, wherein the runner (2) isextended (8) after the connecting point of the connecting duct (3) tothe reaction chamber (4).
 8. A gating system as claimed in any one ofclaims 1-7, wherein the cross-sectional area of the connecting duct (3)has been dimensioned for an influx rate to the reaction chamber (4) of<500 mm/s.
 9. A gating system as claimed in any one of claims 1-8,wherein the connecting duct (3) and the reaction chamber (4) have thesame width.
 10. A gating system as claimed in any one of claims 1-9,wherein the reacting chamber (4) has a square sectional area.
 11. Agating system as claimed in any one of claims 1-10, wherein theconnecting area of the pressure and mixing chamber (5) to the reactionchamber (4) is ≧the cross-sectional area of the connecting duct (3). 12.A gating system as claimed in any one of claims 1-11, wherein the heightof the partition (9) of the pressure and mixing chamber (s) iscalculated according to the formula: height(mm)=30+3×the height of theconnecting duct (3) to the reaction chamber (4).
 13. A gating system asclaimed in any one of claims 1-12, wherein the height of the pressureand mixing chamber (5) is ≧the height of the partition (5) plus theheight of the connecting duct (3) to the reaction chamber (4).
 14. Agating system as claimed in any one of claims 1-13, wherein the volumeof the first part of the pressure and mixing chamber. (5) is ≧half thevolume of the reaction chamber (4).
 15. A gating system as claimed inany one of claims 1-14, wherein the cross-sectional area of the outletduct (6) of the pressure and mixing chamber (5) is ≧ the cross-sectionalarea of the connecting duct (3).